Barack Obama stepped into the crisis over the oil spillage in the Gulf of Mexico yesterday, amid growing criticism that the US administration and BP were failing to get a grip on the potential catastrophe.

The president flew to New Orleans and travelled on by road to Venice, a small fishing town at the mouth of the Mississippi which has become the command post for the rapidly escalating rescue preparations.

As rain poured down upon him, Obama gave his assessment of what he called the "massive and potentially unprecedented environmental disaster" caused by three leaks still uncapped on a BP oilrig south of the Louisiana coast.

He warned that it could take days to stop the leaks, and said he recognised that "people are understandably frustrated and frightened, particularly as people in this region have already been through more disasters than anybody should have to bear."

And he issued a stern message to the British oil giant: "BP is responsible for this leak. BP will be paying the bill."

With the memory of George Bush's delayed appearance in the region after Hurricane Katrina in 2005 clearly in mind, Obama has made sure his administration has been highly visible since the crisis began on 20 April with the explosion at the Deepwater Horizon rig, 50 miles off the coast. The president said he had pursued an "all hands on deck" approach, saying his team had showed a "relentless response to this crisis from day one".

Ken Salazar, the interior secretary, told NBC's Meet the Press that it could take 90 days to drill a relief well that would stop the leaks.

At the current rate, estimated at 200,000 gallons of oil a day, that would mean about 18m gallons of oil will escape, and make the Deepwater Horizon a significantly larger spillage than the Exxon Valdez disaster in 1989, when 11m gallons ended up in the Alaskan sea.

Satellite images analysed by the University of Miami suggest that the area of ocean covered by the slick has tripled over the last few days. The slick is now nine miles from shore, though taking longer than expected to reach land.

In preparation, BP and the Louisiana coastguard have been mobilising fishermen in what could become a huge deployment of boats attempting to hold back the oil. Locals have compared the preparations to those before Dunkirk during the second world war.

About 300 fishermen, many of them of Vietnamese origin, turned up at a high school in Boothville, a small fishing village on the banks of the Mississippi, for safety training by BP before their possible use in the rescue work and clean-up.

The training was staged on the basketball court used by the school team, fittingly called the Oilers, whose logo is an oil derrick.

BP said that 500 fishermen had already been formally contracted to do the work, which will begin as soon as the slick gets close to the fragile ecosystem of marshes which fan out around the Mississippi estuary.

With BP promising to pay boat captains at least $1,200 (£786) a day for their services and the use of their vessels, the scheme, dubbed Vessel of Opportunity, has attracted huge interest. Fishermen from as far afield as Texas, Arizona and Florida have travelled to the area in the hope of securing contracts, though BP has been strict in turning them away in favour of local labour.

"We are acutely aware that this region is being hit for the third time – after hurricanes Katrina and Rita – and our whole idea is to put the money that will be spent back into this community," said David Kinnaird, a BP community outreach officer organising the programme.

He said that local captains would be invaluable in ameliorating the impact of the slick because of their knowledge. "They know where the oil will go, where the tides will push it, where the inland creeks and rivers are. We have to put that experience to work."

The fishermen are not being expected to be able to handle the oil. Instead, they will be trained to place absorbent booms in the water to act as giant sponges, soaking up the oil. The booms will then be picked up and disposed of by BP workers with expertise in dealing with hazardous materials.

Richard Blink, aged 23, whose stepfather owns 35ft and 50ft boats, was one of the hundreds who queued up for more than an hour to attend the training. "I'm hoping they will use me in whatever way they can to help the clean-up," he said. Part of the difficulty facing BP and its volunteer army of fishermen as the programme gets under way will be the weather.

High winds coming from the south-east are continuing to hamper efforts by making it impossible for smaller boats to go out and by whipping the oil up and over booms which have been laid on the ocean in an attempt to hold back the slick.

Forecasts suggest that the winds may swing round in the next few days and push the slick towards Mississippi.

Some predications warn the slick could be carried by the Gulf Stream as far as the Florida Keys and up the east coast of the US.